Telephone circuit



April 7, 1942. G, KNlTTLE A l 2,278,702

l Y TELEPHONE CIRCUIT Filed oct. 18, 1940 251W@ V W l i l I INVENTOR yATTORNEY Patented Apr. 7, 1942 l TELEPHONE CIRCUIT Gale Leonard Knittle,Brookfield, Ill., assigner to American Telephone and Telegraph Company,a corporation of New York Application October 18, 1940, Serial No.361,686

12 Claims. (Cl. 179-42) This invention relates to telephone circuits,and more particularly to subscribers circuits so arranged that anattendant may intercept the call of any one of a number of subscribersin a common locality in the absence of the subscriber.

In business oiices it is a common arrangement to have the subscribersloops of a number of subscribers in the same business organization ordepartment thereof, or in the same physical locality, connect to aso-called key telephone set to enable an attendant to answer the call ofany subscriber who may be away from his desk. In order that such servicemay be available, a key telephone set is provided which has a number ofkeys, one for each subscribers line. A number of signal lamps are`provided, likewise one for each subscribers line. The telephone set, onthe operation of the key corresponding to any line, may be connected tothe line to answer a call in the place of the subscriber who is absent.

The key telephone set is associated with equipment which, in response toa ringing signal, causes the lamp associated with the particular linecalled to flash to apprise the attendant of the fact that av call hascome in. The attendant is not supposed to answer the call unless thelight continues to flash for some length of time, as would be the casewhen the subscriber is not at his desk. If, however, the light continuesto flash, the attendant may operate the key to connect his set to theline which is being called. In order that the attendant may know whenthe call has been answered and, after the call has been answered, thatthe line is busy, the circuit is so arranged that under these conditionsthe signal lamp, instead of flashing, glows continuously. This action ofthe signal lamp takes place regardless of whether the call is answeredby the attendant or by the subscriber himself.

To enable the lamp to iiash it is necessary to provide an interruptingdevice of some sort which ordinarily consists of a train of relays sotimed in their operation as to interrupt the circuit of the lamp about60 times per minute. In addition to this interrupter the equipment isalso provided with a second interrupter which performs a somewhatdifferent function. It is desirable that the lamp shall continue toflash even after the ringing signal has ended, and for this reason theringing relay is locked up to keep the signal lamp flashing until someone answers the call. Under certain conditions, however, particularly atnight when not only the subscribers are not present but no attendant ison duty, it is undesirable that the operation of the ringing relay'should keep the lamp flashing. Therefore an interrupting arrangement isprovided which will interrupt the locking circuit after the expirationof some reasonable length of time such as, for example, from 8 to 15seconds. This involves the provision of another train of relays so thatall together very expensive and elaborate relay circuits are provided inorder to make these interrupting devices available. l

It is the purpose of the present invention to improve and simplify theapparatus of such key telephone units by using thermal contact devicesto perform the interrupting functions above described. These thermalcontacts may consist of bimetallic thermal .elements surrounded byheating coils. When current iiows through the heating coil a givenlength of time, the metallic element is heated with the result that dueto the unequal expansion of the metals, the element is deformed so thatit may either open or close they contact. By properly adjusting thevalue of the current, the number of turns of wire in the coil,

and the design of the bimetallic element, the action of the thermalcontact arrangement to either open or close the circuit may becontrolled to take place within certain time limits. The use of devicesof this nature Will result in greatly simplifying and cheapening theequipment associated with the key telephone unit.

The invention may now be more fully under-r stood from the followingdetailed description thereof when read in connection with the attacheddrawing in which Figure L1 shows a circuit diagram embodying theprinciples of the invention; Fig. 2 illustrates a thermal contact deviceof the type used in connection with the present invention; and Figs. 3and 4 show modifications of the circuits of the thermal contact devicesused in the arrangement of Fig. l.

Referring to Fig. 1, numerals I and 2 designate the tip and ringconductors of a telephone loop incoming from a central oiice or a P. B.X. omce to a subscribers station. At the right the conductors I and 2pass through the key telephone arrangement X to a telephone subscribersset (not shown).

The key telephone unit is associated with a signal lamp L for the lineand adapted to flash about times perr minute in response to a callincoming from the central oflice. For this purpose the lamp iscontrolled by a ringing relay RR. 'Ihe same lamp is also used toindicate the busy condition of the circuit, after the subscriber orattendant has answered the call, by glowing steadily. The circuit of thelamp is controlled under these conditions by an answering relay ARserially included in the conductor 2 so that it receives direct currentfrom the central -o'ice when the subscribers loop is closed. Theattendant who has control of the key telephone unit also is providedwith a key (not shown) by which her telephone set may be connected tothe loop to answer the call in the absence of the subscriber.

In order to eiect the ashing of the lamp L in response to the ringingsignal received from the ringing relay RR, a thermal contact device TCzis used as an interrupter. This thermal contact device is supplied, inthe arrangement shown in Fig. 1, with current from an alternatingcurrent source through a transformer T having a primary winding P and asecondary winding S associated with the thermal contact device TCz. Thecircuit is so arranged that when the relay RR is actuated, the lamp L issupplied with current through the transformer T over the contact of thethermal contact device TCz, and as the thermal contact device is sodesigned as to interrupt this circuit at intervals of about once persecond, the lamp L of course flashes so long as this circuit ismaintained by the ringing relay RR.

The thermal contact device is illustrated in more detail in Fig. 2. Itconsists of a bimetallic element 2 made up of two metals whose rates ofexpansion when heated are diierent. This bimetallic element issurrounded by a heating coil C and when current flows through the coilit heats up the bimetallic element and causes it to be deformed so as toopen the contact D after a definite interval. In so doing it interruptsits own circuit so that current no longer flows through the coil C andthe bimetallic element is allowed to cool and again close the circuitwhen the operation is repeated.

An alternative circuit is provided for the lamp L under the control ofthe answering relay AR, and when this circuit is closed the lamp issupplied with current through the transformer T over a circuit which isnot interrupted by the thermal contact device TCz. As a consequence,under these conditions the lamp glows steadily.

The ringing relay RR is provided with a locking circuit so arranged thatthe relay RR will be locked up and maintained energized after theringing current ceases. This locking circuit is opened when theanswering relay AR is operated and therefore the lamp ceases to flashwhen the call is answered but thereafter glows steadily. However, if acall comes in and no one answers the call, the lamp will continue toiiash indelinitely unless means is provided to prevent it. 'Ihis is acondition that may occur at night when the oil-ice is closed and noattendant is at the key telephone unit to answer the call. It isundesirable to have the lamp thus continue to ash and therefore a secondinterrupter of the thermal contact type is shown at TC1. The contactcontrolled by this interrupter is included serially in the circuit ofthe locking winding of the ringing relay RR. This particular thermalcontact device is designed to open its circuit after a much longerinterval than the device TG2, thus allowing the locking circuit to bemaintained after the cessation of the ringing current for a period ofabout 8 to 15 seconds. If no one answers the call during this period,the thermal contact device TC1 opens the locking circuit of the relay RRand restores the apparatus to normal, so that the lamp L no longerglows. In the arrangement shown in Fig. l the thermal contact device TG1is operated by direct current supplied by battery B1, whereas thethermal contact device TG2 is shown operated by alternating currentsupplied through the transformer T.

Suppose now a ringing signal is sent out from the central ofhce. Usuallysuch a ringing signal consists of a succession of spurts of ringingcurrent continued at regular intervals until somebody answers the callor until the operator decides that the party is not going to answer andpulls down the connection. The first spurt of ringing current operatesthe left-hand winding of the relay RR and causes it to pull up itsarmatures. At its inner right-hand contact a locking circuit iscompleted from the grounded battery B1 through the coil of the thermalcontact device TC1 and over the contact of said thermal contact device,thence through the right-hand locking winding of relay RR, over itsinner front contact and over the back contact of relay RR to ground. Asthe thermal contact device TC1 does not interrupt this circuit for atleast 8 seconds, this locking circuit is maintained between theintervals of the spurts of ringing current and for at least 8 secondsafter the expiration of the last spurt.

During the time that the ringing relay is locked up, a circuit is closedfor the signal lamp L at the outer right-hand contact of relay RR. Thiscircuit extends from the grounded side of the secondary winding S of thetransformer T, over the contact of the thermal contact device TCz, overthe conductor 3, outer right-hand contact of relay RR, over theconductors 4 and 5, and thence through the signal lamp L to the oppositeterminal of the secondary winding S. Alternating current is thussupplied from the A. C. source through the transformer T to the signallamp L. It will be noted, however, that the winding of the thermalcontact device TG2 is bridged across the secondary of the transformerbeyond the interrupting Contact of said device so that alternatingcurrent also ows through the coil. The coil is therefore heated andafter a short interval opens its contact, thereby interrupting thecircuit of the lamp L and also the circuit of the coil itself. The coilnow begins to cool and the thermal element closes its contact when theoperation is of course repeated. The result is that the A. C. currentsupplied to the lamp L is interrupted about-once per second due to theaction of the thermal contact device and the lamp therefore flashes,indicating to the attendant at the key telephone unit that a call hascome in over the line. The line lamp continues to flash withoutinterruption until the thermal contact device TG1 operates to open itscontact and interrupt Vthe locking circuit of relay RR about 8 secondsafter the last spurt of ringing current has been received.

If the subscriber should answer before this interruption takes place,the answering relay AR will be operated to interrupt the locking circuitof the relay RR and cause it to become deenergized, thus opening theflashing circuit of the lamp L at the outer right-hand contact of therelay RR. The operation of the relay AR connects ground over its frontContact, over the conductors 6 and 5, through the lamp L and thencethrough the secondary winding S of the transformer T to ground. Thiscircuit, it will be noticed does not pass through the interruptingcontact of the thermal contact device T02 and consequently the lamp nowglows steadily, apprising the attendant at the key telephone unit thatthe call has been answered by the subscribed.

If, on the other hand, the subscriber delays answering his call untilthe interrupter TC1 has come into play and unlocked the ringing relayRR, the lamp L will cease to iiash and will no longer glow. Ifthereafter the subscriber closes his switchhook contact, therebyactuating `the relay AR, relay AR will cause the lamp to glow steadilyover the circuit controlled by its front contact as just described.Again the attendant at the key telephone unit is apprised of the factthat 4the subscriber has answered by the steady glowing of the lamp. If,however, the subscriber delays unduly in answering the call, the lamp Lwill cease to iiashafter ringing ceases and the attendant, dependingupon her instructions or her judgment as to the exigencies of thesituation, may come in and answer the call by throwing her key andconnecting her set across the conductors I and 2 at the right (notshown) This will result in energizing the answering relay AR and againthe lamp L Will be caused to glow steadily over the circuit closed overthe front contact of the answering relay AR as already described.

It will be obvious, of course, that the thermal contact device TCioperates to unlock the ringing relay RR and open the iiashing circuit ofthe lamp `L after a predetermined interval. This prevents alternatingcurrent being Wasted through the resistance of the lamp L at night, orduring other periods when the key telephone unit is unattended and callscome in which are not answered.

Thermal contact devices such as TC1 and TCz being temperature-controlleddevices will vary somewhat in their time of operation with changes inthe temperature of the room in which they are located. Consequently onhot summer days they Will operate more frequently and after a shorterinterval than in colder weather. This, of course, changes the rate atwhich the lamp L will flash and also may change the interval duringwhich the relay RR remains locked up after the last spurt of ringingcurrent. It is not essential, however, that the lamp L should flashalways at the same rate of speed. The essential thing is that it doesash when a call comes in. Neither is it essential that the thermalcontact device TG1 shall open the locking circuit of the ringing relayRR in exactly 8 seconds or 15 seconds or some other period of time thatmay be set. If, under the conditions under which it operates after theshortest period of time, it takes, for example, 8 seconds to open itscontact, its operation will be satisfactory because the delay of a fewmore seconds in cold'weather will be of little consequence. However, theoperation of these thermal Contact devices may be stabilized as toperiod of operation by enclosing them in the same casing with thetransformer T. Alternating current from the power source is continuouslyiiovving from the primary of the transformer T whether current passesthrough the transformer to the signal lamps or not. The result is thatthe transformer winding is heated to some extent and the temperaturewithin the transformer casing is above that of the'room in which it islocated. The heat from the transformer therefore serves to maintain thethermal contacts ata more uniform temperature under varying weatherconditions.

As shown in Fig. l, the thermal contact device TG1 is operated from adirect current source while the thermal contact device TG2 is operatedfrom an alternating current source. In actual practice, however, theymay be operated by either direct or alternating current. For example, inFig. 3 the thermal contact device TC1 is shown with its contact andwinding 4connected in series with the secondary Winding S of thetransformer T. Therefore the circuit previously traced through thelocking winding of relay RR of Fig. 1 is supplied with alternatingcurrent. The timing of the thermal contact device may be controlled byproper design to cause the actuation of the contact in the desired timeunder the conditions of current supply thus existing. `The lockingwinding of the relay RR, of course, must be designed to hold up thearmature of the relay when supplied with current alternating at 60cycles or whatever rate may characterize the current supply.

Under the conditions just described, with the alternative arrangement ofFig. 3 the circuit of the thermal contact device TG2 may be actuated byalternating current from the power source as shown bythe circuitarrangement of Fig. 1. However, the thermal contact device TCz may alsobe operated by direct current if `the circuit of Fig. 4 is used. VHereit will be seen that the secondary winding S is vomitted and the directcurrent battery B2 is connected in its stead. The direct current fromthe battery ows over the contact of the thermal contact device TC2 andthrough the winding thereof to cause the contact to open and close atthe desired rate. vDirect;

current will now be supplied vthrough the lamp L over the circuitcontrolled by the left-hand front contact of the relay RR to cause thelamp L to iiash in response to a call. When the call is answered thelamp L will be supplied with direct currentfovera circuit from ground,through the battery B2, through lamp L, and thence over conductors 5 andB and the front contact of the answering relay AR to ground. As thiscircuit is independent of the interrupting contact of the thermalcontact device TG2 the lamp will flash continuously under theseconditions.

It will be obvious that the general principles herein disclosed may beembodied in many other organizations widely different from thoseillustrated without departing from the spirit of 'the invention asdefined in the following claims.

What is claimed is:

l. In a telephone system, a subscribers line arranged for theinterception of a call by anattendant, a signal lamp, means responsiveto a call on said line to establish a circuit for flashing said lamp, athermal contact device including a contact in said lamp circuit and athermal element, and means to supply alternating current over saidcontact to said thermal element and to said lamp, whereby said thermalelement is heated by current passing over said contact to open and closesaid contact and thereby cause said'lamp to flash.

2. In a telephone system, a subscribers line arranged for theinterception of a call by anattendant, a signal lamp, means responsiveto a call on said line to establish a circuit for flashing said lamp, athermal contact device including a contact in said lamp circuit and athermal elementVmeans to supply alternating current over said contact tosaid thermal element and to said lamp, whereby said thermal element isheated by current passing over said contact to open and close saidcontact and thereby cause said lamp to ilash, and means responsive tothe answering of said call to disestablish the circuit of said lamppassing over said contact.

3. In a telephone system, a subscribers line arranged for theinterception of a call by an attendant, a signal lamp, a ringing relayresponsive to ringing current to establish a circuit for nashing saidlamp, a locking circuit for said ringing relay, a thermal contact deviceincluding a contact in said lamp circuit and a thermal element heated bycurrent passing over said contact to open and close said contact andthereby cause said lamp to flash, and means responsive to the answeringof the ringing signal on said line to open said locking circuit of saidringing relay and disestablish the circuit of said lamp passing oversaid contact.

4. In a telephone system, a subscribers line arranged ior theinterception of a call by an attendant, a signal lamp, a ringing relayresponsive to ringing current to establish a circuit for ashing saidlamp, a locking circuit for said ringing relay, a thermal contact deviceincluding a contact in said lamp circuit and a thermal element, means tosupply alternating current over said contact to said thermal element andto said lamp,

whereby said thermal element is heated by current passing over saidcontact to open and close said contact and thereby cause said lamp toflash, and means responsive to the answering of the ringing signal onsaid line to open said locking circuit of said ringing relay anddisestabto open and close said contact and thereby cause said lamp toash, and means responsive to the answering of said call to disestablishthe circuit of said lamp passing over said contact and to establish acircuit for said lamp independent of said contact so that said lamp willglow steadily to indicate that the call has been answered.

6. In a telephone system, a subscribers line arranged for theinterception of a call by an attendant, a signal lamp, means responsiveto a call on said line to establish a circuit for flashing said lamp, athermal contact device including a contact in said lamp circuit and athermal element, means to supply alternating current over said contactto said thermal element and to said lamp, whereby said thermal elementis heated by current passing over said contact to open and close saidcontact and thereby cause said lamp to flash, and means responsive tothe answering of said call to disestablish the circuit of said lamppassing over said contact and to establish a circuit for said lampindependent of said contact so that said lamp will glow steadily toindicate that the call has been answered.

'7. In a telephone system, a subscribers line arranged for theinterception of a call by an attendant, a signal lamp, a ringing relayresponsive to ringing current to establish a circuit for flashing saidlamp, a locking circuit for said ringing relay, a thermal contact deviceincluding a contact in said lamp circuit and a thermal element heated bycurrent passing over said contact to open and close said contact andthereby cause said lamp to flash, and means responsive to the answeringof the ringing signal on said line to open said locking circuit of saidringing relay and disestablish the circuit of said lamp passing oversaid contact and to establish a circuit for said lamp independent ofsaid contact so that said lamp will glow steadily to indicate that thecall has been answered.

8. In a telephone system, a subscribers line arranged for theinterception of a call by an attendant, a signal lamp, a ringing relayresponsive to ringing current to establish a circuit for flashing saidlamp, a locking circuit for said ringing relay, a thermal contact deviceincluding a Contact in said lamp circuit and a thermal element, means tosupply alternating current over said contact to said thermal element andto said lamp, whereby said thermal element is heated by current passingover said contact to open and close said contact and thereby cause saidlamp to ash, and means responsive to the answering of the ringing signalon said line to open said locking circuit of said ringing relay anddisestablish the circuit of said lamp passing over said Contact and toestablish a circuit for said lamp independent of said contact so thatsaid lamp will glow steadily to indicate that the call has beenanswered.

9. In a telephone system, a subscribers line arranged for theinterception of a call by an attendant, a signal lamp, means responsiveto a call on said line to establish a circuit for flashing said lamp, athermal contact device including a contact in said lamp circuit and athermal element heated by current passing over said contact to open andclose said contact and thereby cause said lamp to ash, and a secondthermal contact device to operate after a predetermined interval todisestablish the circuit of said lamp passing over the contact of saidrst thermal contact device.

10. In a telephone system, a subscribers line arranged for theinterception of a call by an attendant, a signal lamp, means responsiveto a call on said line to establish a circuit for flashing said lamp, athermal contact device including a contact in said lamp circuit and athermal element, means to supply alternating current over said Contactto said thermal element and to said lamp, whereby said thermal elementis heated by current passing over said contact to open and close saidcontact and thereby cause said lamp to flash, and a second thermalcontact device to operate after a predetermined interval to disestablishthe circuit of said lamp passing over the contact of said first thermalcontact device.

11. In a telephone system, a subscribers line arranged for theinterception of a call by an attendant, a signal lamp, a ringing relayresponsive to ringing current to establish a circuit for flashing saidlamp, a locking circuit for said ringing relay, a thermal contact deviceincluding a contact in said lamp circuit and a thermal element heated bycurrent passing over said contact to open and close said contact andthereby cause said lamp to ash, means responsive to the answering of theringing signal on said line to open said locking circuit of said ringingrelay and disestablish the circuit of said lamp passing over saidcontact, and a second thermal contact device including a thermal elementand contact controlled thereby in said locking circuit, to operate aftera predetermined interval to disestablish said locking circuit andthereby disestablish the circuit of said lamp passing over the contactof said rst thermal contact device.

12. In a telephone system, a subscribers line arranged for theinterception of a call by an attendant, a signal lamp, a ringing relayresponsive to ringing current to establish a circuit for flashing saidlamp, a locking circuit for said ringing relay, a thermal contact deviceincluding a Contact in said lamp circuit and -a thermal element, meansto supply alternating current over said contact to said thermal elementand to said lamp, whereby said thermal element is heated by currentpassing over said contact to open and close said contact and therebycause said lamp to flash, means responsive to the answering of theringing signal on said line to open said locking circuit of said ringingrelay and disestablish the circuit of said lamp passing over saidcontact, and a second thermal contact device including a thermal elementand contact controlled thereby in said locking circuit, to operate aftera predetermined interval to `disestablish said locking .circuit andthereby disestablish thev circuit of said lamp passing over the contactof said rst thermal contact device.

GALE L. KNITTLE.

